This disclosure of Japanese Patent Application No. 2001-203592 filed on Jul. 4, 2001 including the specification, drawing and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to an abnormality diagnosis system and method for an oxygen sensor. More particularly, the invention relates to an abnormality diagnosis system and method which performs a diagnosis on an abnormality of the oxygen sensor caused by its fractured detector.
2. Description of Related Art
In an internal combustion engine which includes an exhaust purification system utilizing a catalyst, it is essential to accurately control an air-fuel ratio of an air-fuel mixture which is combusted in the internal combustion engine such that the catalyst effectively purifies exhaust gas emission. The air-fuel ratio is defined as a weight ratio of the air to the fuel contained in the air-fuel mixture for combustion. In the internal combustion engine requiring the aforementioned highly accurate control of the air-fuel ratio, an oxygen sensor for detecting a partial pressures of oxygen (hereinafter referred to as oxygen partial pressure) in exhaust gas is provided in an exhaust system of the engine. The air-fuel ratio obtained on the basis of the detected partial pressures of oxygen is used for executing feedback control of the air-fuel ratio.
By way of an example, a tubular oxygen sensor utilizing a solid electrolyte will be described. FIG. 1 shows a conceptual structure of the tubular oxygen sensor (hereinafter referred to as an xe2x80x9coxygen sensorxe2x80x9d). As shown in FIG. 1, the oxygen sensor includes a tubular detector that extends into an exhaust passage. An inside of the detector is exposed to the atmosphere whereas an outside thereof is exposed to the exhaust gas that flows into the detector through a sensor cover that surrounds the detector. The detector is made of a solid electrolyte and the inside and outside of the detector are covered with electrodes as shown in FIG. 1. The solid electrolyte is a solid substance that allows movement of an ionized oxygen. For example, zirconia is typically used as the solid electrolyte for the oxygen sensor.
As shown in FIG. 1, a difference of the oxygen partial pressure is caused between the atmosphere (inside of the detector) and the exhaust gas (outside of the detector) which has been isolated via the detector. In this state, the oxygen in a high oxygen partial pressure side (normally the atmosphere side) is ionized and the resultant oxygen ion moves to a low oxygen partial pressure side (normally the exhaust gas side) so as to reduce the difference of the oxygen partial pressure. The oxygen molecule receives four electrons into an ionized state, and releases four electrons into an unionized state. In the course of the movement of the oxygen molecules, electrons move between the electrodes formed on the inside and outside of the detector, generating an electromotive force. Accordingly, the oxygen sensor generates a voltage corresponding to the difference in the oxygen partial pressure between the atmosphere and the exhaust gas.
The oxygen partial pressure of the exhaust gas varies with a change in the air-fuel ratio of a combusted air-fuel mixture. For example, when the air-fuel mixture is combusted at a stoichiometric ratio or in a fuel rich condition, most of the oxygen contained in the air-fuel mixture will be burnt out. As a result, the oxygen partial pressure of the exhaust gas becomes substantially 0. Conversely, when the air-fuel mixture is combusted in a fuel lean condition, an excess amount of oxygen is kept unburned. That is, the oxygen partial pressure of the exhaust gas becomes higher as the air-fuel ratio becomes leaner. Meanwhile, the oxygen partial pressure of the ambient air remains generally constant. Accordingly, the air-fuel ratio of the air-fuel mixture combusted in the internal combustion engine may be obtained on the basis of the output voltage of the oxygen sensor corresponding to the oxygen partial pressure relative to that of the atmosphere.
Various type of oxygen sensors may be employed as well as the aforementioned tubular oxygen sensor. The oxygen sensor including a strip-type detector, the oxygen sensor including a detector made of zirconia, or the like may be applied to the diagnosis system. The aforementioned oxygen sensor is constructed to detect an oxygen partial pressure of the exhaust gas in the same manner as described above. That is, the detector of the oxygen sensor outputs the detection signals in accordance with the difference of the oxygen partial pressure between the exhaust gas and the reference gas. Most oxygen sensors use the atmosphere as the reference gas relative to the exhaust gas in the same manner as in the oxygen sensor shown in FIG. 1.
An internal combustion engine requiring an air-fuel ratio control for the purpose only of a stoichiometric combustion tends to employ an oxygen sensor having its output voltage considerably increased or decreased at a point of the stoichiometric air-fuel ratio. The aforementioned type of oxygen sensor may satisfy the requirement of the stoichiometric combustion in spite of a low resolving power that only indicates whether the air-fuel ratio corresponding to the detected output voltage is richer or leaner than the stoichiometric air-fuel ratio. On the contrary, the internal combustion engine in which the air-fuel mixture is combusted at a wider range of the air-fuel ratio, for example, a lean-burn combustion, the oxygen sensor is required to have a relatively higher resolving power that allows the output voltage to be linearly changed in accordance with the oxygen partial pressure of the exhaust gas.
When the detector of the aforementioned oxygen sensors has a fracture as shown in FIG. 2A, the exhaust gas flows into the detector such that the inside of the detector is filled with the exhaust gas. Accordingly the oxygen partial pressures in the inside and outside of the detector become equal. The oxygen sensor becomes incapable of generating the electromotive force.
When identifying an output pattern in which detection signals indicating no difference of the oxygen partial pressure between the inside and the outside of the detector are continuously generated by monitoring the output of the oxygen sensor, it is determined that the detector is fractured.
The time period taken from the start of the output of the detection signal indicating the lean air-fuel ratio (lean signal) compared with the stoichiometric air-fuel ratio to output the signal indicating rich the air-fuel ratio (rich signal) compared with the stoichiometric air-fuel ratio is constantly measured during the engine operation. It is determined that the detector is fractured when the measured time exceeds a predetermined time period.
The aforementioned diagnosis system, however, may not always detect the fracture of the detector accurately in the case as described below. In the internal combustion engine for a vehicle, a fuel-cut control for temporarily cutting the fuel injection to the internal combustion engine is frequently performed. During the fuel-cut operation, air is supplied to the exhaust passage. As a result, both the inside and the outside of the detector are filled with air. When the fuel injection resumes in the aforementioned state, the exhaust gas resulting from combustion of the fuel may flow into the exhaust passage.
As shown in FIG. 2B, when the detector is fractured in the aforementioned state, there is substantially no difference in the oxygen partial pressure between the inside and the outside of the detector even if the air-fuel ratio becomes rich as the air-fuel combustion resumes. As a result, the oxygen sensor continues outputting the lean signal. In this state, however, a certain period of time is taken for the exhaust gas reaching the outside of the detector to enter into the inside of the detector through the fracture. Therefore, in the state immediately after the stop of the fuel-cut operation, air exists inside of the detector and the exhaust gas exists outside of the detector even if the detector is fractured. This state as indicated in FIG. 2C is similar to the state of the detector with no fracture. As a result, electromotive force is generated by the oxygen sensor caused by the difference of the oxygen partial pressure between the inside and outside of the detector. This oxygen sensor, thus, may generate the rich signal temporarily.
In the diagnosis performed on the basis of the time period for which the lean signal of the oxygen sensor is switched to the rich signal, the system may fail to detect the fracture of the detector, providing a wrong determination that the detection portion is not fractured.
In a generally employed diagnosis system disclosed in Japanese Laid-Open Patent Publication No. 8-21282, monitoring of the sensor outputs for detecting the fracture of the detector is inhibited until a predetermined time period elapses from the stop of the fuel-cut operation. As a result, the detection signal obtained immediately after the stop of the fuel-cut operation is not reflected in the abnormality diagnosis, thus avoiding diagnosis error.
Although the above-described system makes it possible to detect the fracture of the detector with a certain level of accuracy, the diagnosis system is required to be further improved.
For example, according to the abnormality diagnosis as aforementioned, monitoring of the detection signal of the oxygen sensor has to be suspended for a substantially long time period elapsing after the stop of the fuel-cut operation such that the undesirable detection signal is not reflected in the diagnosis. The long suspension of monitoring the sensor output may delay detection of the fracture of the detector, thus failing to cope with the problem caused by the abnormality of the oxygen sensor.
Depending on the operating state of the internal combustion engine, the sensor in the abnormal state owing to the fracture may take an output pattern similar to that of the oxygen sensor in the normal state with no fracture. On the contrary, the oxygen sensor in the normal state may take an output pattern similar to that of the oxygen sensor in the abnormal state. Monitoring of the sensor output for a substantially long period is required in order to distinguish the output pattern that indicates the fractured detector from the output pattern. Therefore, monitoring of the sensor outputs is required to be performed for a substantially long period of time so as to discriminate the abnormal state of the oxygen sensor from the exceptional cases.
The invention thus provides an abnormality diagnosis system and method for an oxygen sensor which accurately determines an abnormality state of an oxygen sensor owing to a fractured detector of the oxygen sensor.
According to an aspect of the invention, a diagnosis system determines an abnormality in at least one oxygen sensor including a detector interposed between a reference gas and an exhaust gas and generates a detection signal in accordance with a difference of an oxygen partial pressure between the reference gas and the exhaust gas. The diagnosis system includes a controller that determines the abnormality of the oxygen sensor owing to a fracture of the detector on the basis of an output pattern of the detection signal of the oxygen sensor. The controller then determines whether the detector has the fracture on the basis of a distribution pattern of the detection signal generated by the oxygen sensor.
As described above, when the detector is fractured, the exhaust gas enters to the inside of the detector exposed to the reference gas through the fracture. Therefore the difference of the oxygen partial pressure between the inside (reference gas) and outside (exhaust gas) of the detector becomes substantially zero. In a particular case, however, the oxygen sensor having a fractured detector may generate a detection signal indicating the difference of the oxygen partial pressure between the inside and outside of the detector in the same way as in the case where the oxygen sensor is in a normal state. Actually, however, such phenomenon is less likely to occur, and the resultant distribution of the detection signal of the oxygen sensor having the fractured detector becomes considerably different from that of the oxygen sensor in the normal state. Even if the oxygen sensor having the fractured detector generates infrequently the detection signal similar to that of the oxygen sensor in the normal state, the abnormality may be appropriately diagnosed on the basis of the distribution of the detection signal of the oxygen sensor.
According to a preferred form of the aspect of the invention, the controller determines that the detector is fractured when it is determined that a ratio of a detection signal indicating the difference of the oxygen partial pressure that is equal to or less than a first predetermined value to the detection signal of the oxygen sensor becomes equal to or greater than a second predetermined value.
As described above, the oxygen sensor having the fractured detector may infrequently generate the detection signal indicating the difference of the oxygen partial pressure between the inside and the outside of the detector. The ratio of the detection signal indicating the difference of the oxygen partial pressure to the detection signal, however, is small. As the distribution of the detection signal of the oxygen sensor having the fractured detector takes a pattern in which distribution of the detection signal concentrates in the region indicating the relatively small difference of the oxygen partial pressure between the inside and outside of the detector. Accordingly, it can be determined that the detector is fractured when the ratio of detection signal indicating the relatively small difference of the oxygen partial pressure to the detection signal is larger than a predetermined value.
According to another preferred form of the aspect of the invention, the controller determines that the detector is not fractured when it is determined that a ratio of a detection signal indicating the difference of the oxygen partial pressure that is equal to or greater than a third predetermined value to the detection signal of the oxygen sensor becomes equal to or greater than a fourth predetermined value.
When the detector is not fractured, distribution of the frequency in generating the detection signal of the oxygen sensor does not concentrate in the region indicating small difference of the oxygen partial pressure. Accordingly, it can be determined that the detector is not fractured when the ratio of detection signal indicating a relatively large difference in the oxygen partial pressure to the detection signal is larger than a predetermined value.
According to another aspect of the invention, a diagnosis system determines an abnormality in at least one oxygen sensor being provided downstream of a catalyst of an exhaust system in an internal combustion engine. The oxygen sensor includes a detector interposed between an atmosphere and an exhaust gas, and generates a detection signal in accordance with a difference of an oxygen partial pressure between the atmosphere and the exhaust gas. The diagnosis system includes a controller that determines the abnormality of the oxygen sensor owing to a fracture of the detector on the basis of an output pattern of the detection signal of the oxygen sensor. The controller inhibits the detection signal of the oxygen sensor from being used for determining the abnormality of the oxygen sensor until a predetermined period of time elapses from a point of time when a fuel-cut operation of the internal combustion engine is stopped, calculates a quantity of oxygen absorbed by the catalyst during the fuel-cut operation, and sets the predetermined period of time to a value that changes depending upon the calculated quantity of oxygen absorbed by the catalyst.
When the oxygen partial pressure of the inside of the detector is increased by a fuel-cut operation, the catalyst starts absorbing. The absorbed oxygen is gradually released into the exhaust gas as the oxygen partial pressure of the outside of the detector (exhaust gas) decreases immediately after stop of the fuel-cut operation. As a result, the oxygen partial pressure of the outside of the detector (exhaust gas) is kept high for a predetermined time period after the stop of the fuel-cut operation. Therefore, it is necessary to inhibit the detection signals of the oxygen sensor generated for the predetermined time period after the stop of the fuel-cut operation from being reflected in the abnormality diagnosis such that the diagnosis is accurately performed on the basis of the output pattern of the oxygen sensor.
The time period for which the high oxygen partial pressure state is kept after the stop of the fuel-cut operation changes depending on the quantity of oxygen absorbed in the catalyst during the fuel-cut operation. The time period for which the generated detection signal is inhibited from being reflected in the abnormality diagnosis is set on the basis of the quantity of oxygen absorbed in the catalyst during the fuel-cut operation. The diagnosis error caused by the state where the oxygen is released from the catalyst immediately after the stop of the fuel-cut operation may be effectively avoided while keeping opportunities for performing the abnormality diagnosis from being reduced.
According to another aspect of the invention, a diagnosis system determines an abnormality in at least one oxygen sensor being provided downstream of a catalyst of an exhaust system in an internal combustion engine. The oxygen sensor includes a detector that isolates an atmosphere from an exhaust gas, and generates a detection signal. The diagnosis system includes a controller that determines the abnormality of the oxygen sensor owing to a fracture of the detector on the basis of an output pattern of the detection signal of the oxygen sensor. The controller inhibits the detection signal of the oxygen sensor from being used for determining the abnormality of the oxygen sensor until a predetermined period of time elapses from a point of time when a fuel-cut operation of the internal combustion engine is stopped, and sets the predetermined period of time in accordance with at least one of a total value of an intake air quantity during the fuel-cut operation and a catalyst temperature.
The more the quantity of air is fed into the exhaust system of the engine during the fuel-cut operation, the more the quantity of oxygen absorbed in the catalyst becomes during the fuel-cut operation. The air quantity can be derived from the total value of the intake air quantity during the fuel-cut operation. As the oxygen absorbing capacity of the catalyst varies depending on the temperature, the quantity of oxygen absorbed in the catalyst during the fuel-cut operation varies accordingly. In the diagnosis system, the time period for which the detection signal of the oxygen sensor is inhibited from being reflected in the abnormality diagnosis is set to a value that changes in accordance with the total intake air quantity during the fuel-cut operation and the catalyst temperature. The resultant time period, thus, can be minimized so as to avoid the diagnosis error.
According to another aspect of the invention, a diagnosis system determines an abnormality in at least one oxygen sensor including at least one detector that isolates a reference gas from an exhaust gas and generates a detection signal in accordance with a difference of an oxygen partial pressure between the reference gas and the exhaust gas. The diagnosis system includes a controller that determines the abnormality of the oxygen sensor owing to a fracture of the detector on the basis of an output pattern of the detection signal of the oxygen sensor. The controller detects a temperature of the detector and inhibits the detection signal of the oxygen sensor from being used for determining the abnormality of the oxygen sensor when the detected temperature is lower than an activating temperature of the detector.
When the temperature of the detector has not reached the activation temperature, the oxygen sensor may fail to generate an accurate detection signal in accordance with the operation state of the internal combustion engine. In some cases, the oxygen sensor having no fractured detector may generate a similar detection signal to that normally obtained when the detector is fractured. In the aforementioned diagnosis system, the detecting signal generated by the oxygen sensor is inhibited from being reflected in the abnormality diagnosis until the temperature of the detector reaches the activation temperature. The diagnosis error owing to the temperature of the detector as described above can be effectively avoided.
According to another aspect of the invention, a diagnosis system determines an abnormality in at least one oxygen sensor including one detector interposed between an atmosphere and an exhaust gas and that generates a detection signal in accordance with a difference of an oxygen partial pressure between the atmosphere and the exhaust gas. The diagnosis system includes a controller that determines the abnormality of the oxygen sensor owing to a fracture of the detector on the basis of an output pattern of the detection signal of the oxygen sensor. The controller determines that the detector is fractured upon generation of the detection signal of the oxygen sensor, which indicates that the oxygen partial pressure of the exhaust gas is higher than that of the air.
The oxygen partial pressure of the combusted exhaust gas never becomes higher than that of the air under no circumstances even if the fuel-cut operation is performed. The oxygen sensor, in some cases, may generate the detection signal indicating that the oxygen partial pressure of the exhaust gas is higher than that of air depending on the operating state of the engine. Accordingly, the fracture of the detector can be easily and accurately determined on the basis of the detection signal which is not expected to be generated by the oxygen sensor in the normal state.